Adapted from Nigella Lawson's Chocolate Orange Cake from Feast. Note due to cooking the lemons this cake has more of a bitter lemon flavour than the fresh zing of lemon. It's not your traditional sweet lemon cake.
Put the lemons in a saucepan with cold water and bring to the boil before reducing the heat and simmering for 2 hours, or until soft. Drain the water and set aside to cool. When cool, cut the lemons in half and remove any large seeds.
Grease a 20cm springform pan and preheat the oven to 180 Celsius (350 Fahrenheit).
Pulp the lemon in a food processor before adding the eggs, baking powder, bicarbonate of soda, almonds and sugar and processing until you have a smooth batter.
Pour the batter into the springform pan and bake for 45 minutes to 1 hour. Check after 40 minutes to ensure the top is not cooking too much, if so simply cover with foil.
Let the cake cool in the tin before removing.
To make the frosting add the butter to the bowl of a mixer fitted with a paddle attachment and beat until pale and creamy. Reduce the speed to it's lowest setting and gradually add the icing sugar. Finally add the pureed blueberries and mix until combined.
To decorate the cake smooth over a crumb coating and refrigerate until set before smoothing over another layer of the frosting.